Pressure-Optimized Band Gap and Enhanced Photoelectric Response of Graphitic Carbon Nitride with Nitrogen Vacancies

Graphitic carbon nitride ($g$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$) shows favorable performance as a photocatalyst and has attracted widespread attention in recent years. As its wide band gap of 2.70 eV limits light absorption in the visible range, many efforts have been made to optimize the band gap. In this report, pressure is used to engineer the band gap and photoelectric response of nitrogen-deficient $g$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ nanoflakes. The band gap of the sample is first narrowed to 2.40 eV due to the introduction of nitrogen vacancies and then further narrowed to 1.70 eV by pressure, which is the lowest value reported in the literature for undoped $g$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$. Accordingly, the photoelectric response increases by nearly 50% because of the enhanced light absorption at high pressure. More interestingly, after depressurization to ambient pressure, the optimized band gap survives with a minimum value of 1.87 eV accompanied by enhanced photoelectric responsivity. In situ synchrotron x-ray diffraction and Raman spectra suggest that the tunable band gap originates from irreversible pressure-induced amorphization with the assistance of vacancies for $g$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$. The collaborative approach of introducing deficiency and pressure treatment adopted here shows the ability to engineer the band gap continuously over a prominently wider region than that for the single band-gap-narrowing technique, and thus, enhances the photoelectric performance for broadened semiconductors.